There's a variety of very common SNPs that people do not convert the vitamin D3 into the 25-hydroxy vitamin D very well, or they do not convert the 25-hydroxy vitamin D into the steroid hormone very well ... I've seen people have to take between 20 to 30,000 IUs/day to even just get a normal 30 or 40 ng/mL blood concentration of 25-hydroxy vitamin D...